Car Audio Systems Explained

Car Audio Systems Explained

Maybe this should be called the "Car Audio myths debunked" page. There's a lot of ridiculous claims 'out there' on the market. I've seen the cigarette pack sized amps that claim 200 watts output, but being an electronics tech, I tend to see through the male bovine excrement of that silliness. Hopefully I can supply some laymen understanding of some car audio terms and explain the meanings of specifications in the back of the stereo and amplifier manuals. Unfortunately, this page kind of turned into geek-speak, so you will have to just read what you want to know about.

CD-ROM Compact Disk- Read Only Memory Data is stored on the CD as a continuous coil starting from the hub, and working it's way out to the edge. They will store about 650 megabytes of digital information. There's about 60,000 tracks per inch on a CD, so you can now see why a CD player tends to skip when bumped. Keeping the laser focused on a track that's 1/1000 the width of a human hair ain't easy! You can tell how much a CD has written by looking at one in the sunlight. Where the "rainbow" effect stops, is where the recorded tracks end. A good CD player will have a few seconds of read ahead memory so it can figure out where it was and start reading the music info again before it stops playing. A skip is actually a data dropout that lasted longer than the read ahead memory had stored ahead. Music data is stored in a format which is read in like this: left channel...right channel...time coding..left channel...right channel...time coding...etc. The track speed is constant, and the CD changes speeds as the laser gets further out on the disk. A CD player is about 1/2 computer, and reassembles the data back into analog sound again. Since it's digital information that's reassembled into music again, frequency response is extremely high, and distortion is all but nil. Also the S/N ratio is extreme. There won't be any hissing or other clues as too how high the CD player is turned up before the speaker cones blow out when the music starts.
DVD Digital Video Disk Also used for data storage as well as movies and music. It's a CD-ROM that has higher density per track as well as about double the number of tracks, and they have two layers of data tracks. The outer layer is kind of like a one-way mirror, and the laser can either focus on it, or past it to the inner layer. It's like stacking two CD-ROM's in the player at the same time. They are also sometimes read from BOTH sides of the disk.
Sampling/Oversampling What a CD-ROM does normally as it reads in the data. It checks the data against stored CRC values to see if it's okay, and checks it again to be sure. Claims as to how many times it over samples, is like selling a car that has an engine as if having one is otherwise something unusual. It has more meaning for a computer data CD than music, because the sampling rate that the music is stored in. (not the same sampling rate as what they're talking about, by the way) A CD player can interpolate missing music data bytes, and you'll never hear it. But, that won't work with computer data. The music sampling is an analog signal that is 'chopped up' into 44,100 pieces per second per channel. Each piece is assigned a digital value of 0 to 256.
Frequency range This is somewhat obvious, but not totally. Most good systems will have 20 to 20,000 hz in the specs. Most humans can hear between 30 and 15,000 cycles per second. hz is an abbreviation for CPS. (cycles per second) Sounds much above 15,000 hz really aren't heard. This is where the "ting" of cymbals lives. Much below 25hz, and you can feel the vibrations, but not really hear them. That's what a good sub woofer is for - making the super lows you can FEEL. And, it takes lots of power to make that low frequency sound.
Wow and Flutter This is a measurement used in tape cassette machines. It has no meaning whatsoever in CD players. It shows how steady the cassette player can run the tape past the heads. Wow is unsteady speed, and flutter would be caused by misshapen capstan rollers or the tape wobbling vertically past the head as it runs past. A cassette worth much should have a very low values for these measurements. But... it will increase with the player's age as the rubber rollers and friction clutches get old.
Signal to Noise Ratio This is a measurement for radio receivers, although sometimes it's used in CD audio measurements too. It's based on db which stands for Decibels. It's named after Alexander Graham Bell. The Bel is a logarithmic value, and one Bel divided by ten, is one decibel. It's not really related to anything in particular, but is a measurement of the RATIO of one value based on another. If a reference number is 1, then 20 db higher than 1 would be 10. 30 db higher would be 100 and so on. One also has to know what the reference signal is to be of any real use. Most radio receivers are based in a signal measured in so many db SINAD. (Signal Including Noise And Distortion) For car audio, any stereo specs better than: S/N Ratio = 60 db, stereo separation = 35db, Image Rejection = 40 db is good. (higher numbers are better) Radio receiver sensitivity is based on dbm. (see below)
Decibel A unit of measurement based on logarithmic numbers. It's used because human's have a non linear way of responding to the environment. A candle burning in a dark room is seen as a certain brightness. Light another, and it doubles the brightness. Now you have to light two more to brighten it again. Sound is the same way. And... in the case of amplified sound such as your stereo, the power level must increase about 10 times for each doubling of the perceived sound. A certain sound, will have to double to be heard as louder, and the power will have to be 10 times higher in the amp. Going from 0.1 watt to 0.2 watts isn't much level change, but 0.1 watts to 1 watt will be heard as double the sound level. Going from 0.5 watts to 1 watt isn't very much, but 0.5 watts to 5 watts is about double the sound level. This is where logarithmic numbers get used, and the reason really good stereo amps are high wattage power pumping units. A 100 watt amplifier is NOT twice the sound pressure level as a 50 watt amp! Twice the power, yes, but not necessarily twice the sound level. There are a few standards as to the db zero setting to measure from. dbm used in radio transmitter/receivers is 1 milliwatt into a 600 ohm load. More signal then 0 dbm is a positive number, and less signal than 0 dbm is a negative number. A good radio receiver measures -120 dbm.
Distortion This is a measurement of the audio signal's wave form. This has more meaning for amplifiers than anything else. CD players are actually decoders for the digital information stored on a CD-ROM, and distortion from a CD is all but nonexistent.
Total Harmonic Distortion Similar to the above. It's done on expensive 'scopes, and measures how messed up the audio wave form gets when compared to the input test signal. The lower the number, the better it is.
Clipping Part of a distortion problem, and what the harsh sound is when you turn an amp up beyond what it can handle. Audio, such as bass from a bass guitar, produces a nice sinusoidal (rounded) type signal. Clipping is when the peaks of the signal gets "flattened" at the top and bottom. This sounds like crap from the speakers, and is a sign an amplifier is being overdriven. In a high powered sound system, you would have to use an oscilloscope to see the amp's signal, because an overdriven speaker can sound much the same way. The cone will overshoot and start hitting the frame and bottom out in the magnet's air gap. Also, if the voice coil gets too hot, the varnish insulation will burn, and then drag against the magnet's air gap. This is usually why a "blown" speaker isn't a good thing. (unlike a "blown" Chrysler 426 Hemi, which is a good thing)
Watts Voltage times current. Simple as that. Example: 12 volts x 5 amps = 60 watts... 120 volts x 0.5 amps = 60 watts. And here's where some of the horse puckey claims come in. An amplifier feeds more or less alternating current to the speakers. The audio wave form isn't pure AC, but is a modulating and changing wave form. DC Ohm's law doesn't really cover speakers because AC is measured in impedance and reactance. Because of the reactance of the moving coil through the magnet, it gets a bit complex to figure out amplifier and speaker watts via formulas. Since most of the power in music is in the low frequencies, (bass) this is what is typically used. The formula for power is: P=EI Cosine Phase Angle. P (power) = E (voltage) x I (current) x phase angle in degrees. Simple ain't it? A good MOSFET 100 watt amp running a 4 ohm speaker at the full 100 watts, that's 4 ohms Impedance, not DC ohms, will put about 24 volts AC at about 5 amps into the speaker. Phase angle is pretty much everywhere in the music's AC wave, so it's hard to calculate, and is usually considered 90 degrees. As you can see, the little cigarette pack sized "200 watt" amp really isn't possible with just a 12 volt supply, and no inverters. A good amp will have inverter circuitry to get the AC voltage up where it's needed. And, that's why a REAL 200 watt RMS amp will need welding wire from the battery, cooling fans, and is the size of a small suitcase. Most amps are rated total power, so divide the watt rating by the number of channels it has. i.e. A 200 watt, 4 channel amp is 50 watts per channel... A 100 watt, 2 channel amp is 50 per channel... etc.
RMS Root Mean Square. The equivalent DC 'work' value an AC voltage can produce. The 120 volts RMS AC at your house outlet, is actually about 170 volts peak to peak. This is a measurement that's only seen on an oscilloscope, and is the actual peaks of the wave form on the scope's screen. A useless measurement, as the extreme upper and lower parts of the AC wave doesn't do any work. The RMS value draws a line at the 0.707 part af the wave form, and this is where the current has actual work value. BUT, this peak to peak measurement is what a lot of auto sound amps use as it's power rating!! It won't anymore move the speaker cone at this peaked value, than the HP number on a dyno caught at the end of it's swing will turn your Suburban's tires against the ground. If an amplifier doesn't rate itself on the box in RMS numbers listed as to what they used for a test load, keep on shopping. (i.e. 40 watts RMS into 2 ohms) And, most cheap amplifier makers use a higher frequency into a 1 or 2 ohm test COIL, and not necessarily a speaker, for the watt ratings. That's their flimsy way of getting it past the Federal Trade Commission.
Bridged Tying channels together to make one high power channel. Usually when the amp used used as a subwoofer driver.
Ferrofluid-Cooling Originally devoloped for the aerospace industry, ferrofluid was designed to cool turbines. It's a gelatinized oil with metalized particles embedded within, and the magnetic impulses of the speaker coil makes it circulate. Found on very expensive speakers.
MOS-FET Metal Oxide Semiconductor Field Effect Transistor Any good amp will have a string of these power transistors for each channel stuffing those watts into your speakers. These are very efficient transistors compared to normal ones, and are used to keep the excess heat to a minimum. A well designed MOS-FET transistor will amplify anything from DC to microwave with little distortion.
Line level input/Output Usually RCA type plugs/jacks. These are the best way to run a system's units into an amp. There's more distortion added by a radio/cassette/CD player's own small amplifier circuits at the speaker outputs, then there is at these jacks. It's a low level audio that's taken before the IC amp circuits in a car stereo, and have to least amount of noise to add to the main amp's input. These are the same low level in/outputs as your home stereo unit. Picture of RCA low level cables:
Floating Ground System Doesn't really mean "floating" at all, just that the speaker outputs are NOT common with ground. This is the type that have a "+" and a "-" wire for each speaker. A lot of stereos use a Kenwood or Motorola inline Integrated Circuit amp chip. These are powerful chips, output each 1/2-phase of the signal to the speaker wires, and have about 7.5 Volts DC on each speaker wire to ground. Do NOT hook up the "-" wires to ground, or the speakers AND the amp IC will go up in flames. Literally.
Common Ground System The speaker "-" wires are common with the vehicles frame ground. Most cheapie stereos, and some OEM units have this setup. It's cheaper for the auto manufactures, because they can just run one wire to each speaker, and ground the other side. The cheaper stereos use less cost IC's and transistor circuits. Don't even try to mix up a common ground stereo with a floating ground system. Unless you have a good, charged fire extinguisher, that is.

Crossovers

Two Way/Three Way Speakers Speakers with extra midrange and tweeter speakers built in. They have crossovers, and the extra speakers are mounted coaxially above the main woofer cone. They work good to add the needed mid to high range sounds to a sound system.
Tweeter A speaker that reproduces only the frequencies above 5,000hz to 10,000hz
Midrange A speaker that reproduces mid frequency range sounds between 400hz and 5000hz.
Woofer A speaker that reproduces the low frequency range from about 30hz to 500hz.
Subwoofer A speaker and/or amp combo that reproduce only the frequencies below about 300hz, and may respond as low as 10hz.
Equalizer/Passive Equalizer These are frequency selective units that come in different "bands" and are found in 3, 5, 7, 10, 12, and 15 band units. Each band is a different range of frequencies and boost or cut the audio by several decibels. The only thing is... the output is typically common, which means all the speakers in the system will be trying to reproduce the modified music. Look for one that has different outputs for subwoofers etc. Dedicated speakers and crossovers are usually better to use. A passive equalizer is just an un-amplified type that uses low level inputs and outputs. (RCA jacks) Some passive units can handle high level speaker in/outs.
Speaker Phasing This is an important one for a good sound system. Speakers are marked "+" and "-" for a reason. It isn't because they are DC polarity sensitive, it refers to which direction the cone will move with a given signal polarity. If a certain signal makes one cone move out, you don't want the other side of your Suburban's speaker cone pulling in. This causes a null due to the opposite speaker cone making a "hole" in the air inside the vehicle at the same time the correct wired speaker cone is making a pressure wave. Note the drawing:
Ohms A speaker is a linear electric motor, and is an AC device. It's measured in ohms of Impedance instead in resistance because of it's reactive, moving coil. If you use an ohm meter to measure a 4 ohm speaker, you will read anything from 2.5 to 6 ohms. This is normal, because an AC signal will "see" the circuit as the correct 4 ohms of impedance, even though the DC powered ohm meter might not. It does not mean the speaker is badly made. If a speaker is dead, it might measure infinite on it's terminals. Most digital meters will show OL which means "Open Lead", and it means the voice coil is burned in two. If this is the case, tear the speaker apart, and use the magnet to hold your children's school drawings to the refrigerator door, or as a nail pickup tool. It can't be fixed. Sometimes an intermittent speaker has fatigued flex wires that are broken.
Series/Parallel Methods of wiring multiple speakers together. When you wire speakers in parallel, the load resistance/impedance value decreases. Each speaker will carry part of the load. When you mix up different speaker impedances, the lower ohm speakers will be louder as they will use more of the supplied power to the circuit. All the speakers in the circuit will divide the power up. 100 watts supplied to two speakers means 50 watts to each. 100 watts into 3 speakers, will be about 33.3 watts each. (assuming all are the same ohm rating) Use the watt ratings on the speakers, and divide them up as to the number of speakers in the stereo channel circuit for the total power each channel will be able to withstand. Tweeters are usually a high impedance unit, and can be as high as a few hundred ohms. Subwoofers with their own built in amps, have an input impedance of several thousand ohms, so don't figure them in as a normal speaker when calculating the power handling. The drawing is a diagram of how to calculate the total ohm impedance of a speaker circuit. If you have a few speakers together, and they are a total of only an ohm or so, don't panic. Most amp manufactures figure on a 1 to 2 ohm speaker load as normal anyway. And, when the speaker cone is moving, the counter EMF that builds up with the voice coil's movement in the magnet, will change the impedance due to changing phase angles. But, 3 speakers should be the limit for each amp channel to be safe. Series wired speakers aren't a good idea, and if one croaks, the rest go silent with it, like Christmas lights when one burns out on the string. This creates an instant open load to the amp. Also, amps work better with lower impedance loads, so avoid series wiring.
L-pad 'L' is short for Inductance in the electronics industry, and is a measurment for coils. They are measured in Henrys, and typically millihenrys or microhenrys. An L-Pad is an 8 - 10 ohm coil with a center tapped "wiper" arm that glides around the coil inside. The audio circuit's "+" and "-" wires are put on each end of the coil as if it's a speaker. The speaker's "+" terminal is hooked to the center tap, and it's "-" terminal is hooked to the "-" of the wire on the L-Pad. As the center tap (wiper) gets closer to the "+" side as the L-Pad is turned, the speaker will get louder. An L-Pad can also be wired as a fader control, by hooking the "+" wire from the amp to the center tap, and the front & rear speaker's "+" to the ends of the coil. Wire the speaker's "-" wires together. Turning the L-Pad will transfer audio level between the speakers. An L-Pad for audio use will be rated in watts.
Fader Control A form of L-pad, except they have two taps on the coil inside leading to one center terminal. Some are high capacity carbon resistance elements instead of a coil. They are for fader control only, and make poor audio level controls like the L-Pad.
70 Volt Line Well, this may not come up in car audio, but some high powered public address (PA) amps have this type of output. If you have one you are planning on using in your Suburban, you might wonder what this output is for. It's designed for running speakers over long runs of wire in buildings, and uses 70 Volt line transformers on each speaker to convert the high voltage audio signal back down into a normal PA speaker's 8 to 16 ohms. The audio signal runs about 70+ volts, (hence the name) so it can travel a long ways in the wire, and can instantly fry normal stereo speakers if hooked directly to this type of amp output. Don't do it!!
Input Polarity Protection A large capacity diode installed reversed across the incoming power circuit inside the amp. Normally, it doesn't do anything. But, if the amp's supply power is wired backwards, or if the car is jumpstarted with reversed cables, the diode conducts all positive current straight to ground, blowing the fuse. This protects any circuits beyond the diode. Always follow the fuse rating. Use too big a fuse, and the diode can vaporize, letting everything else go with it, or the circuit trace leading to the diode can burn in two. Bad news for you, good news for a repair shop.
Overtemperature Protection A thermistor mounted on the heatsink that trips a shutdown circuit when the amp starts running too hot. Much better than transistors burning up, or unsoldering themselves from the circuit board. Shorted speaker lines will cause a trip, as well as running the amp too hard. If it shuts down, there's a problem, as most amps will start to distort the audio long before it gets overheated from overdriving. It will happen if the amp doesn't have enough air flow too.
Turn-On Thump A good designed amplifer will have a delay circuit during power up that will eliminate this "whump" sound as the amp is turned-on . It's caused by the speaker cones getting shoved out hard as the circuits come alive. Cheaper amps and stereo units lack the 75 cents worth of parts this takes to fix this annoying problem. There are add on "turn-on thump" eliminators on the market to get rid of this problem. It won't hurt the speakers, but it is annoying.
Remote Turn On/Automatic Power Switching A circuit for remotely powering up the amplifier. A good stereo unit will have a remote output wire just for this function. That way, the amp can be in the back, and not be powered on all the time. A power antenna output wire on the stereo will work for this too.
Gold Plated Terminals Not just advertising hype. Think about 500 year old gold Doubloons brought up from the ocean floor. Shiny huh? That's because pure gold will not tarnish... ever. You don't want connections corroding in a damp environment due to cheap connectors.
DIN Mounting/DIN Standard Stands for: Deutsches Institut für Normung. An institution of standards based in Germany. They set the standards for a lot of the computer industry as well as other manufacturing.

A few classic Suburban sound ideas


Here's where I planted the amp. It's under the glove box.	Another view of the amp's location. It's almost not noticeable from above.

Here's where I mounted the CD player. The "clock" is a WWII aircraft altimeter. Great for mountain driving!!	Click for a better view of the diagram of how the CD player is mounted solid under the dash.

Lots of room in the side walls in the back end for the Sony 6x9's...	...and in the thick doors as well. 6 1/2" Blaupunkts went here.

Highs...	...and lows!!

Page last updated: 3/4/00

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